This invention relates to processes for forming a barrier layer on metal surfaces. The invention finds particular application in conjunction with forming barrier layers that resist hydrogen permeation, resist corrosion by liquid metals, resist wear and abrasion, and resist surface attacks by corrosive media.
Heretofore, various methods have been used to provide barriers or coatings that are resistant to the permeation of hydrogen. For example, the permeation of hydrogen and its isotopes, such as tritium, through the walls of nuclear reactors and their components, has been a problem that has never been completely solved. One method of reducing the rate of hydrogen permeation has been to form an intermetallic aluminide, specifically nickel aluminide, as a coating on certain metals to slow the loss of hydrogen. A drawback to this approach is the high temperature and long time required to form the barrier, and the effect of this time-temperature relationship upon the mechanical characteristics (tensile strength, fatigue strength, etc.) of the underlying alloy.
Another method was to use the naturally grown oxide scale on nickel-chromium alloys. However, these oxides are continually reduced by the hydrogen atmosphere. The continuous presence of water vapor is required in order to reform the oxides. Analogous hydrogen barriers have been used to slow the permeation of hydrogen from Stirling engines which contain high pressure hydrogen as the working fluid.
The corrosion of containment walls of systems that use liquid metals as the working fluid, e.g. nuclear reactor heat transport systems and sodium heat pipes, is another difficult problem that has never been totally solved. Nickel aluminide coatings have been used with some success, but the very high temperature required to form the aluminide can weaken the base material. In most of these systems, it is necessary to maintain the oxygen level extremely low in order to prevent oxygen enhanced corrosion reactions with component elements of the alloys, resulting in an eventual loss of containment.
The present invention relates to a new and improved technique for forming surface barriers which overcomes the above-referenced problems and others, and to the structures produced by such technique.